Since carbon fibers have low ductility and are brittle, they easily become fuzzy as a result of mechanical friction and also lack wettability with respect to matrix resins. Consequently, it is difficult to fully demonstrate the superior properties of carbon fibers when used as reinforcing materials. In order to improve on this, carbon fibers have been treated in the prior art with a sizing agent. Various compounds are known to function as sizing agents in this manner. For example, Japanese Unexamined Patent Application, First Publication No. Sho 50-59589 discloses the application to carbon fibers of a solvent solution of a sizing agent composed of polyglycidyl ether (to be abbreviated as “Sizing Agent 1”). In addition, Japanese Unexamined Patent Application, First Publication No. Sho 61-28074 describes the formation of an aqueous emulsion of a bisphenol type of polyalkylene ether epoxy compound using a small amount of emulsifier (to be referred to as “Sizing Agent 2”), and this is known to be applied to carbon fibers.
In addition, one example of a fiber-reinforced composite material is a molded article composed by molding a resin composition containing reinforcing material of carbon fiber and a matrix resin. Epoxy resin is widely used as the matrix resin of such fiber-reinforced composite materials. In addition, numerous other resins, including unsaturated polyester resins, vinyl ester resins, acrylic resins and other radical polymerized resins, are also used in addition to epoxy resin.
When obtaining a carbon fiber-reinforced resin composition composed of carbon fibers as the reinforcing material and a matrix resin, examples of methods for impregnating the carbon fibers with the matrix resin include a prepreg method in which carbon fibers are arranged unidirectionally on a matrix resin thinly coated on mold release paper, and a dipping method in which carbon fibers are dipped into a resin bath.
In addition, a carbon fiber-reinforced resin composition can also be obtained by impregnating a matrix resin into a carbon fiber woven fabric after the carbon fibers are processed into a woven fabric with a weaving machine. Examples of methods for obtaining this type of carbon fiber woven fabric-reinforced resin composition include a prepreg method in which carbon fiber woven fabric is layered on a matrix resin thinly coated on mold releasing paper, and a dipping method in which carbon fiber woven fabric is dipped into a resin bath.
In order to mold a stable fiber-reinforced composite material of high quality on an industrial scale, it is necessary that impregnation of carbon fiber bundles composed of several thousand filaments and matrix resin is able to be carried out easily and completely in the impregnation process in which carbon fibers are impregnated with matrix resin.
However, since carbon fibers have low ductility and are brittle, they easily become fuzzy as a result of mechanical friction and also lack wettability with respect to matrix resins. Consequently, it is difficult to allow carbon fibers used as a reinforcing material to fully demonstrate their superior properties as described above. In order to improve on this, carbon fibers used as reinforcing materials of fiber-reinforced composite materials have conventionally been treated with a sizing agent. That is, as a result of treating carbon fibers with a sizing agent, in addition to improving the handling ease of the carbon fibers, their wettability with respect to matrix resin is also improved. As a result, the quality of molded articles composed of a fiber-reinforced composite material using carbon fibers as a reinforcing material is improved. Various compounds are used as sizing agents in this manner.
For example, a sizing agent that uses a polyglycidyl ethers, etc. (to be referred to as “Sizing Agent 3”), is proposed (see Japanese Examined Patent Application, Second Publication No. Sho 57-15229). In addition, a sizing agent having as essential components an epoxy resin and a condensation product of the alkylene oxide adduct of bisphenols with an unsaturated dibasic acid, and the alkylene oxide adduct of phenols selected from a monocyclic phenol and a polycyclic phenol (to be referred to as “Sizing Agent 4”) is proposed (Japanese Unexamined Patent Application, First Publication No. Sho 53-52796, Japanese Unexamined Patent Application, First Publication No. Hei 7-197381). In addition, various other sizing agents are also proposed including a sizing agent composed of an epoxy resin, the alkylene oxide adduct of a monocyclic or polycyclic phenol, an unsaturated dibasic acid or its ester-forming derivative and polyester condensation product of the alkylene oxide adduct of bisphenols, which has an acid value of 40 or less (to be referred to as “Sizing Agent 5”) (Japanese Unexamined Patent Application, First Publication No. 10-60779).
The aforementioned Sizing Agent 3 has the advantage of superior impregnation and interface adhesive force at the time of use. Sizing Agent 4 is able to improve adhesion with matrix resin, and particularly unsaturated polyester resin. In addition, in the case of using epoxy resin for the matrix resin, Sizing Agent 4 is superior in terms of being able to alleviate the problem of the conventional art of fluctuations in physical properties of carbon fiber-reinforced resin compositions due to fluctuations in curing conditions. In addition, Sizing Agent 5 remains stable over time and has superior unwinding properties, while also exhibiting satisfactory adhesion with unsaturated polyester.
However, since Sizing Agent 1 uses a solvent solution, it had the disadvantage of poor industrial handling and safety considerations in comparison with aqueous types when used to treat carbon fiber sizing agents. In addition, although Sizing Agent 2 improves on the disadvantages of Sizing Agent 1, it was found to have the disadvantages indicated below depending on the selection of the emulsifier. That is, since the emulsion stability of the epoxy compound is inadequate in the case in which the emulsifier is a nonionic surfactant, during treatment for applying a carbon fiber sizing agent, a portion of the emulsion is destroyed resulting in the occurrence of defects by sizing agents and other problems in the carbon fiber production process.
In addition, in the case of an anionic surfactant having an electrical charge that is capable of improving emulsion stability, when the anionic surfactant is that in which the counter ion is an alkaline metal or alkaline earth metal ion, these alkaline metal or alkaline earth metal ions end up contaminating the fiber-reinforced composite material resulting in problems such as decreased thermal stability.
On the other hand, in the case of an anionic surfactant in which the counter ion is an ammonium ion, since this type of surfactant has reactivity with epoxy groups, it gradually reacts with epoxy groups of the adhered sizing agent after adhering to the carbon fibers as sizing agent. As a result, the problem occurred in which the carbon fibers become hard, causing prominent changes over time. Moreover, in the case of a cationic surfactant which also has an electrical charge and is similarly capable of improving emulsion stability, there is the disadvantage of being more expensive than anionic surfactants.
Moreover, since Sizing Agent 3 does not have adequate adhesion with unsaturated polyester resin, vinyl ester resin, acrylic resin and other radical polymerized resins, it is unsuitable for using these resins as the matrix resin of a carbon fiber-reinforced resin composition. In addition, although Sizing Agents 4 and 5 are superior to Sizing Agent 3 in terms of adhesion to radical polymerized resins, the level of adhesion is still not adequate. Consequently, there are still problems with using these resins as the matrix resin of a carbon fiber-reinforced resin composition.